Infectious laryngotracheitis is caused by the infection of ILTV. ILTV infects avians such as chickens, pheasants, peacocks and turkeys. Characteristic features of onset in chickens include the appearance of respiratory symptoms, elevation of body temperature and anorexia and the like, serious coughing, expectoration of sputums. When egg-laying chickens are infected, the rate of laying eggs starts to decrease about four days after the onset of the disease, and takes about one month before normal egg-laying returns. Furthermore, increases in death rate due to mixed infection of ILTV and other pathogens have been reported, and infectious laryngotracheitis inflicts huge economic losses on the poultry industry.
For the prevention of infectious laryngotracheitis, dried live vaccines or frozen live vaccines from attenuated vaccine strains have conventionally been used. However, the effect of immunization varies with the breeding environment, the breeding density, the inoculation method and the like. Furthermore, there are also risks that vaccination may cause slight symptoms in the respiratory tract and faulty methods or amounts of inoculation may lead to the onset of the disease. In some areas, there are reports of diseases caused by the reverted pathogenicity of vaccine strains, and thus the development of safe and effective vaccines is being sought.
In order to overcome the problems, recently, vaccines comprising as an active ingredient a recombinant virus vector have been developed by the recombinant technology. With regard to ILTV, the use of fowlpox virus (hereinafter referred to as FPV) as the virus vector has been investigated and it is commercially available in the USA (BIOMUNE, trade name VECTORMUNE FP-LT(+AE)).
ILTV is a causative virus of infectious laryngotracheitis. ILTV is one of the herpesviruses and the virus genome consists of a double stranded DNA comprising about 160,000 base pairs. There are known the thymidine kinase gene (Griffin et al., J. Gen. Virol. 71:841, 1990), gp60 gene (Kongsuwan et al., Virus Genes 7:297-303, 1993), the capsid p40 gene (Griffin et al., Nucleic Acids Res. 18:366, 1990), the glycoprotein B (gB) gene (Poulsen et al., Virus Genes 5:335-347, 1991; Griffin et al., J. Gen. Virol. 72:393-398, 1991; U.S. Pat. No. 5,443,831), the glycoprotein C (gC) gene (Kingsley et al., Virology 203:336-343, 1994), the RR2 gene (Griffin et al., J. of General Virol. 70:3085-3089, 1989), the UL32 gene (International Patent Publication WO98/07866), and the like.
The open reading frame of gB gene of ILTV has a full-length of 2613 bp (873 amino acids), and it is reported in U.S. Pat. No. 5,443,831 etc. that a recombinant FPV having the full-length gene integrated therein exhibits an effect as a vaccine.
Poxviruses such as FPV rapidly grow in the host and after expressing the antigen protein, they are completely expelled from the host's immune system. However, as immunity is memorized and boosted after expelling the virus, the poxvirus is suitable for use as the host for vaccine. On the other hand, herpesviruses such as turkey herpesvirus (hereinafter referred to as HVT) do not rapidly grow in the host, and latent infection lasts for a long time. During this latency, herpesviruses continue to stimulate the immune system of the host. The use of recombinant HVT as a virus vector using such a characteristic is being investigated.
Although PCT Japanese National Publication No. 4-501658 (EP 434721) described that a recombinant HVT comprising HVT, into which an ILTV antigen gene has been inserted, has been constructed in fact the recombinant HVT has not been constructed.
In addition, in Japanese Unexamined Patent Publication No. 2001-000188, a recombinant HVT, into which two genes, i.e., a full-length gB gene of ILTV and an UL32 gene have been inserted, has been constructed. It has been confirmed that this recombinant HVT expresses in vitro a protein corresponding to the inserted gene using a immunofluorescent assay method, but an effect as a vaccine has not been confirmed.